1. Field of the Invention
The present invention relates to a varistor and, in particular, to a laminated chip varistor capable of applying uniform soldering onto terminal electrodes only of the laminated chip varistor and to a production method thereof.
2. Description of the Related Art
Recently, electronic equipments has been miniaturized. For example, in electric parts driven by a small voltage such as IC, it is necessary to protect them from abnormal voltage, and a chip-type varistor has been generally used.
For chip parts, silver is used as the terminal electrodes. However, because the silver electrode is eroded by solder, nickel plating, etc., is applied onto the silver outer electrode to solder it. Also, in order to improve the soldering property, tin or tin-lead plating is further applied onto the nickel plating, etc.
A varistor layer constituting the laminated chip varistor is mainly composed of ZnO. Because ZnO is a semiconductor, when above-described nickel plating and tin or tin-lead plating is carried out by electrolytic plating, the ceramic portions of the varistor layer is also plated.
In order to prevent this, a high-resisting layer is formed on the surface of a ceramic element, which becomes a chip-type varistor, by dipping a glass composed of an oxide of Si, B, Bi, Pb, Ca, etc., or a high-resisting layer is formed by placing a mixture mainly composed of oxides of Si, Fe, Al, Ti, Sb on the surface of the ceramic element followed by burning (see, Unexamined Japanese Patent Publications (kokai) Hei-8-31616, Hei-8-124720, and Hei-8-153607).
However, in the glass coating and the surface treatment process with the oxide as described above, the work is complicated and further there is a problem that lowering of the yield occurs by attaching the glass or the surface oxide to portions other than the necessary portions, which results in increasing the cost.
Also, it has been found that when an unevenness exists on the surface of the above-described ceramic element, an electric field is liable to be concentrated to the projected portions and these portions are plated to spread undesirable plating to the surrounding surface of the element. Accordingly, it becomes necessary to remove the unevenness of the surface of the element to form the flat surface, thereby the occurrence of the undesirable flow of plating is prevented.